Earnings per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Numerator for diluted and basic earnings per share:
Net income	$ 32,275	$ 25,084	$ 59,574	$ 62,985
Denominator - share:
Basic weighted-average shares (in shares)	29,985	31,017	30,032	31,003
Effect of dilutive securities (in shares)	1,315	1,243	1,399	1,395
Diluted weighted-average shares (in shares)	31,300	32,260	31,431	32,398
Earnings per share:
Basic (in dollars per share)	$ 1.08	$ 0.81	$ 1.98	$ 2.03
Diluted (in dollars per share)	$ 1.03	$ 0.78	$ 1.90	$ 1.94